Inkjet printers are commonplace in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes a printing medium, such as paper.
An inkjet printer produces a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes “dot locations”, “dot positions”, or pixels”. Pixels vary in size, the smaller the dot in the rectilinear array, means that more dots can be printed per inch of the printed medium. Smaller dots result in a more accurate rendition of the image and this in turn results in greater definition of the image. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink of specific size or from a combination of different sized dots.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more print cartridges each having a printhead with a nozzle member having ink ejecting nozzles. The carriage traverses over the surface of the print medium. The width of the carriage varies among the different printers. For any line of print, the carriage may make more than one traverse and utilize a varying number of nozzles. An ink supply, such as an ink reservoir, supplies ink to the nozzles. The nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed and to the physical properties of the ink and the print media.
In general, the ink is housed in a vaporization chamber with a tube leading to a nozzle exposed to the print media. Small drops of ink are ejected from the nozzles through orifices by rapidly heating a small volume of ink located in the vaporization chambers with small electric heaters, such as small thin film resistors. The small thin film resistors are usually located adjacent the vaporization chambers. Heating the ink causes the ink to vaporize and eject ink in the connecting tubing through the nozzle orifices. Specifically, for one dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor of a selected vaporization chamber. The resistor is then heated and in turn heats a thin layer of ink located within the selected vaporization chamber, causing explosive vaporization, and, consequently, a droplet of ink is ejected from the nozzle and onto a print media. The vacuum created as the ink droplet is ejected from the nozzle acts as a suction pump to draw more ink into the vaporization chamber.
Gas is also held in solution in liquids such as ink. The colder the ink, the greater the amount of gas that is held. As the ink increases in temperature, the solubility of the gas decreases, and it leaves the solution in the form of bubbles. The higher the temperature, the more bubbles are formed, and they form at a faster rate. If the temperature reaches a sufficiently high temperature the solution itself may reach its boiling point and also form a gas. The bubbles from either source choke the nozzles and cause deterioration in the quality of the image on the print media.
Temperature also controls the uniformity of the drop size of the ejected ink. The heat from the resistors causing the explosive vaporization in the chamber also causes the size of the drop of ink formed in the chamber to vary. There is an optimal temperature operating range for printheads using inks, in particular, pigmented inks. If the temperature is too low the ink droplets formed will be smaller and have a lower drop-weight than that required for good image quality. As the temperature rises, the drop-weight of the ink droplet will rise. The variation in drop weight varies with the ink being used. These variations in drop-weight will cause visible hue shifts in the printed image.
The temperature will be high if the resistors fire a number of times in a short period of time. Also, if the length of the current pulse to the resistor is longer than a pre-determined limit. As the carriage traverses in a print swath, various heater elements in the array are activated. If the traverse is narrow, the mean temperature at the beginning of the traverse will be similar to the mean temperature at the conclusion, and the effect of temperature on the pass will be consistent for all ink droplets projected onto the print media. If the swath is wide, and more heater elements are activated, the mean temperature at the end of the pass may be considerably higher than at the beginning. The difference in temperature from the beginning of the pass to the end of the pass could result in variation in the drop-weight of ink droplets on the same pass. This would result in hue variation on the one line of print.
Generally, the temperature of the printhead is approximated by two measurements, the thermal sense resistor [TSR], and the digital temperature sensor [DTS]. The DTS is a point sensor located at the top of the die near a firing heating element. While this sensor more accurately reflects the temperature at that point, it does not give an accurate temperature for other heating elements on the die.
The TSR is an approximation of the mean temperature of the printhead die. It is not located adjacent to any particular heating element and reflects the temperature of the die after heat has moved from the heating elements to the TSR. There is, therefore, a delay in the temperature reported by the TSR. The longer the printhead fires, the greater will be the temperature recorded by the TSR. When the printhead has been idle, for example, at the beginning of a print pass, the temperature recorded by the TSR will be low as the die will be cool. The droplets produced at this time will be of low drop-weight. As the pass continues and the number of heating elements firing has increased, the temperature at the TSR will have increased and the drop-weight of the ink droplets will have increased. The difference in temperature from the beginning of the pass till the end of the pass will affect the size of the ink droplets across the pass.
To minimize the effect of temperature variance from the beginning of printing to another point in the printing process, a warming device may be employed. A warming device is used to raise the temperature of the printhead. The printhead assembly may include a means to control the electrical current to the firing resistors so that their temperature is below the threshold required to eject an ink drop. This device could be a power field effect transistor [FET]. The device provides a capability to warm the printhead assembly to the desired temperature before or during printing operations. The process is called “trickle warming” because the printhead assembly allows only a trickle of energy to flow to the firing resistors. The printhead assembly temperature rises until the desired temperature is reached and the warming device is then shut off. However, these systems do not effectively control increases in the mean temperature of the die, and hence, cannot optimize the temperature operating range of the die.
Therefore, what is needed is a method to control and decrease the temperature difference of the printhead from the beginning of the swath to the end, when necessary. What is also needed is a printing system that controls the temperature of the die by measuring and incorporating the temperatures of the heating elements in the printhead die and using these temperatures to preheat the die. What is additionally needed is a feedback loop to turn off these heating elements once an optimal operating temperature has been attained. What is further needed is a system that will produce a more uniform dot pattern on each print pass of the printhead, an improved quality of ink droplet size and a better image quality with a die temperature controller.